﻿/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode head=null,tail=null;
        int carry=0;
        while(l1!=null||l2!=null){
            int n1=l1!=null?l1.val:0;
            int n2=l2!=null?l2.val:0;
            int sum=n1+n2+carry;
            if(head==null){
                head=tail=new ListNode(sum%10);
            }else{
                tail.next=new ListNode(sum%10);
                tail=tail.next;
            }
            carry=sum/10;
            if(l1!=null){
                l1=l1.next;
            }
            if(l2!=null){
                l2=l2.next;
            }
        }
            if(carry>0){
                tail.next=new ListNode(carry);//最后进位
            
        }
return head;
    }
}